U.S. patent number 4,304,603 [Application Number 06/177,010] was granted by the patent office on 1981-12-08 for glass-ceramic compositions designed for radomes.
This patent grant is currently assigned to Corning Glass Works. Invention is credited to David G. Grossman, Richard W. Waldron.
United States Patent |
4,304,603 |
Grossman , et al. |
December 8, 1981 |
Glass-ceramic compositions designed for radomes
Abstract
The instant invention is drawn to glass-ceramics especially
designed for the fabrication of radomes, wherein cordierite is the
predominant crystal phase but which also contain minor amounts of
cristobalite, magnesium-aluminum titanate, and rutile, having
overall compositions consisting essentially, expressed in weight
percent on the oxide basis, of about
Inventors: |
Grossman; David G. (Corning,
NY), Waldron; Richard W. (Big Flats, NY) |
Assignee: |
Corning Glass Works (Corning,
NY)
|
Family
ID: |
22646806 |
Appl.
No.: |
06/177,010 |
Filed: |
August 11, 1980 |
Current U.S.
Class: |
501/9; 501/136;
65/31; 65/33.7; 65/33.9 |
Current CPC
Class: |
H01Q
1/42 (20130101); C03C 10/0045 (20130101) |
Current International
Class: |
C03C
10/00 (20060101); H01Q 1/42 (20060101); C03B
032/00 (); C03C 003/22 (); C03C 003/30 () |
Field of
Search: |
;65/31,33 ;106/39.8 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
D G. Grossman, "Fortification of Radume Glass-Ceramics", Paper
Presented at Fourteenth Symposium on Electromagnetic Windows,
Georgia Institute of Technology, Jun. 21-23, 1978..
|
Primary Examiner: Fisher; Richard V.
Attorney, Agent or Firm: Janes, Jr.; Clinton S.
Claims
We claim:
1. A glass-ceramic suitable for the fabrication of radomes with
porous surface layers having a depth of about 0.005"-0.015", said
glass-ceramic exhibiting a loss tangent at 25.degree. C. and
8.6.times.10.sup.9 Hz no greater than about 0.00035, a dielectric
constant at 25.degree. C. and 8.6.times.10.sup.9 Hz no more than
about 6, a modulus of rupture after fortification in excess of
30,000 psi, a coefficient of thermal expansion
(25.degree.-300.degree. C.) of less than about 45.times.10.sup.-7
/.degree. C., and wherein cordierite constitutes the predominant
crystal phase along with minor amounts of cristobalite,
magnesium-aluminum titanate, and rutile, said glass-ceramic having
an overall composition consisting essentially, expressed in weight
percent on the oxide basis, of about 15-18% MgO, 21-25% Al.sub.2
O.sub.3, 48-53% SiO.sub.2, 9.5-11.5% TiO.sub.2, and 0-1% As.sub.2
O.sub.3.
2. A glass-ceramic according to claim 1 consisting essentially, as
calculated from the batch of:
3. A method for making a glass-ceramic suitable for the fabrication
of radomes exhibiting a loss tangent at 25.degree. C. and
8.6.times.10.sup.9 Hz no greater than about 0.00035, a dielectric
constant at 25.degree. C. and 8.6.times.10.sup.9 Hz no more than
about 6, a modulus of rupture after fortification in excess of
30,000 psi, and a coefficient of thermal expansion
(25.degree.-300.degree. C.) of less than about 45.times.10.sup.-7
/.degree. C. which comprises the steps of:
(a) melting a batch for a glass consisting essentially, expressed
in weight percent on the oxide basis, of about 15-18% MgO, 21-25%
Al.sub.2 O.sub.3, 48-53% SiO.sub.2, 9.5-11.5% TiO.sub.2, and 0-1%
As.sub.2 O.sub.3 ;
(b) simultaneously cooling the melt at least below the
transformation range thereof and forming a glass shape of a desired
configuration therefrom;
(c) heat treating said glass shape at a temperature between about
1000.degree.-1300.degree. C. for a period of time sufficient to
cause the in situ crystallization of cordierite as the predominant
crystal phase along with minor amounts of cristobalite,
magnesium-aluminum titanate, and rutile;
(d) contacting the surface of said crystallized shape with an
alkaline solution to leach out cristobalite therefrom;
(e) removing said alkaline solution from the surface of said
crystallized shape;
(f) contacting the surface of said crystallized shape with an acid
solution to leach out residual glass therefrom;
(g) removing said acid solution from the surface of said
crystallized shape; and then
(h) sequentially repeating steps (d)-(g) a sufficient number of
times to produce a porous surface layer having a depth of about
0.005"-0.015".
4. A method according to claim 3 wherein said heat treating
consists of first heating said glass shape to about
800.degree.-900.degree. C. for about 1-6 hours to cause the
development of nuclei therein and then heating said shape to about
1150.degree.-1300.degree. C. for about 4-12 hours to cause the
growth of crystals on said nuclei.
5. A method according to claim 3 wherein said batch consists
essentially of:
Description
BACKGROUND OF THE INVENTION
A radome is a structure which envelopes and protects a radar
antenna from the environment and, desirably, causes very little
interference with the signal. Missile radomes are ogival or
bullet-shaped shells which shield the antenna from high air
velocities, rain, etc. Radomes fabricated from fiber glass have
proven satisfactory for missiles operating at low velocities. With
high velocity missiles, however, where greater surface heating,
larger loading forces, and more severe rain erosion are
encountered, radomes manufactured from ceramic materials have been
utilized. Ceramic radomes must have the capability of being shaped
through grinding to a very specifically-defined prescription, this
prescription being designed to insure that the resistance to the
signal of the antenna is uniform in every direction.
For over 20 years Corning Glass Works, Corning, N. Y. has
manufactured radomes for radar guided missiles from a glass-ceramic
material marketed as Corning 9606. That product is highly
crystalline with cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2)
constituting the predominant crystal phase with minor amounts of
cristobalite (a polymorph of SiO.sub.2), rutile (TiO.sub.2), and a
phase until recently believed to be magnesium dititanate
(MgO.2TiO.sub.2) being present also. An approximate analysis of the
material, expressed in weight percent on the oxide basis, is
reported below:
______________________________________ SiO.sub.2 56.1 Al.sub.2
O.sub.3 19.7 MgO 14.9 As.sub.2 O.sub.3 0.4 TiO.sub.2 8.9
______________________________________
To be useful as a radome, a material must comply with a complex
matrix of mechanical, electrical, thermal, and forming properties,
several of the most important of which are discussed in the
following text.
The candidate glass-ceramic should exhibit a low loss tangent. The
loss tangent defines the quantity of energy absorbed by a material
from radiation passing therethrough. High loss tangents have the
effect of reducing the range of the radar. Furthermore, not only is
the magnitude of the loss tangent significant, but the level
thereof should be reasonably stable over the range of temperatures
to be encountered by the material. Corning 9606 demonstrates a loss
tangent at 8.6.times.10.sup.9 Hz of 0.00030 at 25.degree. C.
The wall thickness of a radome is dictated by three factors: loss
tangent, the dielectric constant of the material, and the
particular wavelength of radiation being employed. Interference
with the signal will be at a minimum if the thickness of the wall
is a multiple of one-half a wavelength. The dielectric constant
affects the velocity of the radiation and, hence, the wavelength
thereof in the glass-ceramic. When the loss tangent is kept
minimal, the thickness of the wall can be increased with decreasing
values of dielectric constant. Also, as with loss tangent, the
dielectric constant ought not to vary greatly as the temperature of
the radome rises. Corning 9606 has a dielectric constant of 5.5 at
room temperature (25.degree. C.) and 8.6.times.10.sup.9 Hz, and
that value is substantially independent of temperature up to about
750.degree. C. As noted above, dielectric constants greater than
5.5 would require somewhat thinner wall thicknesses and such would
result in greater internal heating of the apparatus within the
radome.
The radome material must exhibit high mechanical strength to
support attachment to the missile, to survive the vibration which
occurs during launch and flight, and to aid in overcoming thermal
stress, the highest stresses being thermal in origin. Resistance to
thermal shock is directly related to the mechanical strength, the
elastic properties, and the coefficient of thermal expansion of a
material. In general, the lower the coefficient of thermal
expansion of a material, the greater will be its resistance to
thermal shock. Corning 9606 displays an average coefficient of
thermal expansion (25.degree.-300.degree. C.) of about
57.times.10.sup.-7 /.degree. C.
Because the newer missiles fly at higher velocities and under more
severe conditions, an improved radome material is demanded which
will demonstrate greater thermal shock resistance than Corning
9606, while exhibiting mechanical, electrical, and forming
properties similar to those of Corning 9606.
U.S. Pat. No. 2,920,971, the basic patent in the field of
glass-ceramics, discloses the production of glass-ceramic articles
as involving the controlled crystallization of precursor glass
articles through a carefully-defined heat treatment thereof. Hence,
the formation of a glass-ceramic body comprehends three general
steps: first, a glass-forming batch of the proper composition is
melted; second, the melt is simultaneously cooled to a temperature
at least below the transformation range thereof and a glass body of
a desired geometry shaped therefrom; and, third, the glass body is
subjected to a predetermined heat treatment at temperatures above
the transformation range to cause the glass to crystallize in situ.
Frequently, this third step is divided into two parts. Thus, the
glass body will initially be heated to a temperature at or somewhat
above the transformation range and held thereat for a sufficient
length of time to cause the development of nuclei in the glass.
Subsequently, the nucleated glass body is heated to a higher
temperature, often above the softening point thereof, and
maintained thereat for a period of time sufficient to effect the
growth of crystals on the nuclei. This two-step practice
customarily results in a more homogeneously crystallized material
wherein the crystals are more uniformly sized.
The microstructure, the general characteristics, and the method for
making glass-ceramic articles are discussed in considerable detail
in U.S. Pat. No. 2,920,971, and that patent is specifically
referred to for a fuller understanding of those features.
A very significant advantage which radomes fabricated from
glass-ceramics possess, when compared with those fashioned through
slip casting or other forming method for ceramic materials, is a
high degree of chemical and structural homogeneity. Thus, the
precursor glass can be melted to a very fine homogeneity and then
dropped into a spinning mold to form the basic ogival article.
Furthermore, deformation during crystallization in situ is very
slight in contrast to that taking place during sintering of a
ceramic body. Accordingly, grinding the radome structure to the
demanded prescription is more easily accomplished than with a
sintered ceramic body.
To impart the required mechanical strength to Corning 9606 radomes,
the bullet-shaped structures, after grinding to the required
prescription, are subjected to what has been termed a fortification
treatment. That treatment comprises subjecting the glass-ceramic to
a sequential base-acid leaching process. Thus, the radome is
initially contacted with (normally immersed into) an alkaline
solution and thereafter, after removing the alkaline solution, it
is contacted with (immersed into) an acid solution. That series of
steps may be repeated several times in order to achieve the desired
effect. As a matter of convenience and economics, a boiling aqueous
NaOH solution has constituted the alkaline environment and an
aqueous, room temperature H.sub.2 SO.sub.4 solution has provided
the acid conditions. The base and acid were customarily removed via
rinsing in tap water.
The improvement in strength is deemed to result via healing surface
flaws in the body. This phenomenon is due to the cristobalite being
leached out of the microstructure (cristobalite is several times
more quickly dissolved in hot NaOH solution than is cordierite).
The acid acts upon the little residual glass left in the
glass-ceramic body. After fortification, Corning 9606 demonstrates
an average modulus of rupture of about 35,000 psi. A somehwat
porous surface layer is developed which protects the radome body
from surface abuse encountered in use that would reduce its
strength. Some care must be exercised in carrying out the
fortification process, however. Thus, excessive treatment causes a
reduction in strength. Although the reason for this reduction has
not been rigorously studied, it has been postulated that
overstretching may lead to the development of new flaws in the body
surface or simply that too much material is removed therefrom.
Empirical observation has determined that a porous surface layer
having a depth of about 0.005"-0.015" appears to yield the most
desirable strength properties.
In general, glass-ceramic articles containing cordierite as the
predominant crystal phase, but with little or no cristobalite, will
demonstrate mechanical strengths, as defined through modulus of
rupture measurements of less than 20,000 psi, whereas those
cordierite-containing articles with a minor, but significant,
amount of cristobalite will evidence modulus of rupture
measurements in excess of 30,000 psi after fortification. X-ray
diffraction analysis and electron microscopy have indicated that
Corning 9606 contains about 10% by volume cristobalite.
OBJECTIVE OF THE INVENTION
The principal objective of the instant invention is to device
glass-ceramic compositions for the fabrication of radomes which
exhibit electrical properties similar to those displayed by Corning
9606 to permit the same design parameters for the radar system to
be used, which can be fortified to mechanical strengths in excess
of 30,000 psi, and which will manifest a lower coefficient of
thermal expansion than Corning 9606.
SUMMARY OF THE INVENTION
We have found that the above objective can be achieved in
compositions falling within the following, narrowly-defined ranges.
Thus, glass-ceramics capable of being formed into radomes
exhibiting loss tangents at 25.degree. C. and 8.6.times.10.sup.9 Hz
no greater than about 0.00035, dielectrric constants at room
temperature (25.degree. C.) and 8.6.times.10.sup.9 Hz no more than
about 6, modulus of rupture values after fortification in excess of
30,000 psi, and coefficients of thermal expansion
(25.degree.-300.degree. C.) of less than about 45.times.10.sup.-7
/.degree. C. can be produced from precursor glasses consisting
essentially, expressed in weight percent on the oxide basis, of
about 15-18% MgO, 21-25% Al.sub.2 O.sub.3, 48-53% SiO.sub.2,
9.5.11.5% TiO.sub.2, and 0-1% As.sub.2 O.sub.3, crystallized in
situ via heat treatments at temperatures between about
1000.degree.-1300.degree. C. The preferred heat treatment
contemplates nucleation within the temperature interval of
800.degree.-900.degree. C. for 1-6 hours followed by
crystallization at 1150.degree.-1300.degree. C. for 4-12 hours.
Cordierite comprises the predominant crystal phase with minor
amounts of cristobalite rutile, and a phase which, although
previously believed to be magnesium dititanate, is now deemed to be
better termed magnesium-aluminum titanate, being observed. Thus, it
has been recognized that complete solubility exists between
Mg.sub.2 Ti.sub.2 O.sub.5 and AlTiO.sub.5. Moreover, examination of
the crystals via X-ray emission spectroscopy has indicated the
presence of approximately 7.5% Al.sub.2 O.sub.3. These factors have
led to the conclusion that the crysrtals are a magnesium-aluminum
titanate.
PRIOR ART
U.S. Pat. No. 2,920,971 discloses the production of glass-ceramic
articles wherein cordierite constitutes the predominant crystal
phase. In truth, Example 15 thereof is essentially the composition
of Corning 9606.
U.S. Pat. No. 3,275,493 describes glasses having compositions
within the ranges of 10-22% MgO, 30-40% Al.sub.2 O.sub.3, 40-57%
SiO.sub.2, and 0.5-6% As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3
which, when heat treated in a particular manner, will develop
integral crystalline surface layers thereupon wherein cordierite is
the principal glass.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Table I reports several compositions, recorded in terms of parts by
weight on the oxide bases as calculated from the batch,
illustrating the parameters of the invention. Because the sum of
the individual components totals or approximately totals 100, for
all practical purposes the tabulated values may be deemed to
reflect weight percent. The actual batch ingredients may comprise
either the oxides or other compounds, which, when melted together,
will be converted into the desired oxides in the proper
proportions. In the succeeding exemplary compositions, the batch
materials consisted of highly pure MgO, Al.sub.2 O.sub.3,
SiO.sub.2, TiO.sub.2, and As.sub.2 O.sub.3. The latter ingredient
performed its customary function of a fining agent.
The batch ingredients were compounded, dry ballmilled together to
assist in achieving a homogeneous melt, and then placed into
platinum crucibles. After covering, the crucibles were introduced
into a furnace operating at about 1600.degree. C. and the batches
were melted for about six hours with stirring. Thereafter, the
melts were cast into steel molds to form slabs having dimensions of
about 6".times.6".times.1/2" and those slabs transferred
immediately to an annealer operating at about 750.degree. C.
TABLE I ______________________________________ 1 2 3 4 5
______________________________________ MgO 15.8 16.0 16.9 17.8 13.3
Al.sub.2 O.sub.3 23.2 21.3 22.2 22.2 28.5 SiO.sub.2 49.9 51.6 49.8
48.9 47.1 TiO.sub.2 10.7 10.7 10.7 10.7 10.7 As.sub.2 O.sub.3 0.4
0.4 0.4 0.4 0.4 ______________________________________
The slabs were introduced into an electrically-heated furnace, the
temperature therein raised at 200.degree. C./hour to 820.degree.
C., that temperature held for about two hours to induce nucleation,
the temperature again raised at 200.degree. C./hour to about
1260.degree. C., that temperature maintained for about eight hours
to grow crystals on the nuclei, and the electric current to the
furnace cut off and the slabs allowed to cool to room temperature
retained within the furnace. This latter practice has been termed
"cooling at furnace rate" and averages about 3.degree.-5.degree.
C./minute.
Samples were cut from the glass-ceramic slabs for measurement in
the conventional manner of coefficient of thermal expansion,
dielectric constant, and loss tangent. Other samples having
dimensions useful in conducting modulus of rupture measurements
were cut from the slabs and subjected to six cycles of the
above-described base-acid fortification treatment. Thus, the
samples were first immersed into boiling aqueous 5% NaOH solution
and held therein for 25 minutes. After rinsing in cold tap water,
the samples were immersed into an aqueous 5% H.sub.2 SO.sub.4
solution at room temperature, retained therein for 10 minutes, and
then rinsed in cold tap water. That sequence of steps was repeated
six times. Microscopic examination of a cross section cut through
the samples indicates a porous surface layer having a depth of
about 0.010"-0.015". Modulus of rupture measurements were conducted
on the fortified samples in the conventional manner. Table II lists
the measured results for coefficient of thermal expansion (coef.
exp.) over the range of 25.degree.-300.degree. C. expressed in
terms of .times.10.sup.-7 /.degree. C., the dielectric constant
(D.C.) at 25.degree. C. and 8.6.times.10.sup.9 Hz, the loss tangent
(L.T.) at 25.degree. C. and 8.6.times.10.sup.9 Hz, and the modulus
of rupture (MOR) of the fortified samples in terms of psi.
TABLE II ______________________________________ 1 2 3 4 5
______________________________________ Coef. Exp. 40.4 35.7 40.4
30.2 23 D.C. 5.64 5.66 6.15 L.T. 0.00029 0.00032 0.0003 MOR 32,130
30,030 31,200 34,600 18,690
______________________________________
The criticality of composition is apparent from an examination of
Tables I and II. Hence, Example 5, having a composition just
slightly outside the inventive ranges, is not sufficiently
mechanically strong to serve as a radome. X-ray diffraction
analysis of Example 5 indicated the essential absence of
cristobalite therefrom, whereas like analyses of Example 1-4
demonstrated the presence of cristobalite crystallization therein.
In all of the examples, cordierite comprised by far the predominant
crystal phase accompanied with minor amounts of magnesium-aluminum
titanate and rutile. Yet, Examples 1-4 evidence high mechanical
strengths coupled with the necessary electrical properties for
radome use and a coefficient of thermal expansion considerably
below that of Corning 9606.
Because it represents the best overall combination of melting and
forming properties, crystallization capabilities, along with the
desired mechanical, electrical, and thermal characteristics,
Example 1 is deemed to constitute the most preferred embodiment of
the inventive compositions.
* * * * *